1. Field of the Invention
The present invention relates to a display apparatus having light emitting elements driven by an active matrix method.
2. Description of the Related Art
Display devices using light emitting elements, such as organic EL elements, for use in various applications, including televisions, cell phone displays, and the like, have been proposed.
Generally, organic EL elements are current driven light emitting elements and, unlike a liquid crystal display, require, as minimum, selection transistors for selecting pixel circuits, holding capacitors for holding charges according to an image to be displayed, and drive transistors for driving the organic EL elements as the drive circuit as described, for example, U.S. Pat. No. 5,684,365 (Patent Document 1).
Heretofore, thin film transistors of low-temperature polysilicon or amorphous silicon have been used in pixel circuits of active matrix organic EL display devices.
The low-temperature polysilicon thin film transistor may provide high mobility and stability of threshold voltage, but has a problem that the mobility is not uniform. The amorphous silicon thin film transistor may provide uniform mobility, but has a problem that the mobility is low and threshold voltage varies with time.
The non-uniform mobility and instable threshold voltage appear as irregularities in the displayed image. Consequently, for example, Japanese Unexamined Patent Publication No. 2003-255856 (Patent Document 2) proposes a display device in which a compensation circuit of diode connection is provided in the pixel circuit.
The provision of the compensation circuit described in Patent Document 2, however, causes the pixel circuit to become complicated, resulting in increased cost due to low yield rate and low aperture ratio.
As such, for example, U.S. Patent Application Publication No. 20050206590 (Patent Document 3) proposes a method for correcting the threshold voltage of the drive transistor by charging a parasitic capacitance of the organic EL element and reducing the number of transistors used in the pixel circuit.
In the pixel circuit described in Patent Document 3, it is necessary to use an n-type thin film transistor as the drive transistor, and the use of an amorphous silicon thin film transistor is envisaged as the n-type thin film transistor.
The amorphous silicon thin film transistor, however, poses a problem that the threshold voltage is shifted by gate voltage stress.
Further, the pixel circuit described in Patent Document 3 has a configuration in which the anode terminal of the organic EL element is connected to the source terminal of the drive transistor, and a capacitor element for detecting the threshold voltage is provided between the gate and source of the drive transistor. In this configuration, the threshold voltage of the drive transistor is held by the capacitor element by applying a predetermined fixed voltage to the gate terminal of the drive transistor to apply a detection current and charging the parasitic capacitance of the organic EL element by the detection current.
Therefore, in order to charge the parasitic capacitance without causing the organic EL element to emit light, it is necessary to set the source terminal voltage Vs of the drive transistor (anode terminal voltage of the organic EL element) lower than emission threshold voltage Vf0 of the organic EL element, as illustrated in FIG. 20. Source terminal Voltage Vs of the drive transistor is determined by the magnitude of the threshold voltage of the drive transistor (minimum value Vthmin to maximum value Vthmax of the threshold value), as illustrated in FIG. 20, so that, when the threshold voltage is shifted by the gate voltage stress, accurate detection and normal correction of the threshold voltage will become impossible and the quality of a displayed image will be degraded. In FIG. 20, VB denotes the fixed voltage applied to the gate terminal of the drive transistor, and ΔVth denotes the magnitude of the variation in the threshold voltage of the drive transistor.
Consequently, Japanese Unexamined Patent Publication No. 2006-227237 (Patent Document 4) proposes a method for preventing a threshold voltage shift of the drive transistor by applying voltage Vg lower than source voltage Vs of the drive transistor to the gate terminal to apply a reverse bias to the drive transistor immediately before a reset period in which data held in the pixel circuit is reset.
The magnitude of gate voltage Vg applied to the gate terminal of the drive transistor when displaying an image depends on the image, and the amount of shift in the threshold voltage of the drive transistor varies with the magnitude of gate voltage Vg. In contrast, the reverse bias period and magnitude of reverse bias voltage in Patent Document 4 are common to all pixels, so that the method can not handle the deviation in threshold voltage of drive transistors and the variation in shift amount of threshold voltages arising from the displayed image. Then, once the shift in the threshold voltage of the drive transistor has started out due to insufficiency of reverse bias, the threshold voltage will shift at an accelerated pace. That is, for the method described in Patent Document 4, it is difficult to prevent the threshold voltage shift of the drive transistor in a case in which the displayed image is updated over a long period of time.
In view of the circumstances described above, it is an object of the present invention to provide a display apparatus capable of preventing threshold voltage shifts of drive transistors and stably correcting threshold voltage variations of the drive transistors over a long period of time.